The term "immersion oil" is a familiar technical term in microscopy and does not signify oils in the ordinary sense, but embraces general immersion media which are suitable for immersion optics.
Oil immersion optics are very common in modern microscopy and in allied fields, since they permit a higher resolution and light intensity as well as a substantially less spherical aberration than dry optics or water immersion optics. The immersion oil, which completely fills the space between objective and cover glass or preparation to be investigated, forms a part of the optical system and therefore must be adapted to this as precisely as possible. The refractive index n.sub.D of the immersion oil should be adapted at least to the glass of the objective front lens, i.e. it should be about 1.500-1.525 at 23.degree. C. According to agreement of microscope manufacturers and corresponding regulations of the German Institute for Standards, an immersion oil should fulfil the following requirements with respect to the refractive indices n.sub.D and n.sub.e and the dispersion .nu..sub.e : n.sub.D (23.degree. C.)=1.515, n.sub.e (23.degree. C.)=1.518.+-.0.0004 and .nu..sub.e =44.+-.5. Further, the immersion oil should have good UV-transmissivity, should be as fluorescence-free as possible and should not attack glass, synthetic object carriers, lens cement and most of the samples to be investigated. Moreover, the immersion oil should be colourless, should be as odourless as possible, should not be hydroscopic, should have a viscosity which permits an easy handling, should have no harmful effect on the user and should exhibit a constant composition, i.e. in particular no optical change should occur by the influence of light, air, temperature and the like.
The hitherto known immersion oils consist for the most part of mixtures of mineral oils, paraffin oils and/or polyisobutylenes with compounds having a high refractive index such as, for example, polychlorinated biphenyls or hydrogenated terphenyls (U.S. Pat. No. 3,929,667). Further, an immersion oil based on thioethers is known from European Patent Application Publication Number 63684. Polychlorinated biphenyls, however, attack synthetic materials (e.g. synthetic object carriers). Further, they are known to be toxic and environmentally dangerous and are therefore only used when it is absolutely necessary because of their favourable fluorescence properties. On the other hand, the remaining previously known immersion oils have other disadvantages, for example they have a moderate to slight, but still troublesome, fluorescence in UV-light and a relatively poor transmissivity, they form streaks or they dissolve the lens cement.